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KND founding father
 
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Stupid owner needs help!

Good morning, I am in need of a new brain and a lot of help (or maybe a little...)

I have an '82 SC that has, up until recently caused me absolutely zero problems. Recently, through a convergence of stupidity and bad luck, jumper cables were reversed while attempting to start my car. They made contact for only a second or less, but immediately following that, if I start the car, it runs for only a second or two and then shuts off. I was given a CIS unit to swap with mine thinking that it would be the first to go. After swapping the units, I experienced the same think... After a bunch of other tests, here is what I've found;

I have no voltage to the socket for the fuel pump relay
The fuel pump fuse is good
The fuel pump relay is good
When I lift on the air flow plate in the airbox (Key in the on position) no fuel pump sound

It appears that after a few days worth of testing, that I have a fuel delivery problem, and more than likely a fuel pump problem.

Whaddya think?

Any help would be greatly appreciated.

With kind regards,
-E
Old 09-12-2007, 04:52 AM
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It is unlikely that the car would start and run for 1-2 seconds if the fuel pump is fried. But based on the other info your post I would agree that you have a fuel problem.

I assume you do not have a set of fuel pressure gages?

Have you considered pulling the fuel pump relay and jumpering it to power the fuel pump directly?
Old 09-12-2007, 06:19 AM
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If the car starts promptly (and then dies) I think your fuel pump is probably OK. It is more likely that you fried something electronic in the control circuitry than the pump itself.

I just spent a while looking at the wiring diagram for the fuel pump circuit in the Bentley manual.

Interestingly it appears that the fuel pump relay closes to shut the pump OFF in conditions when it should not be running.

If the relay is energized the pump only gets power when you are cranking the engine, and will stop after the engine fires and you let the ignition key spring back to the "run" position.

It appears that the relay can be energized (to cut fuel) by 3 other items in the circuit:

(1) the air flow plate microswitch is closed, either because the engine is not running and the plate has not lifted off of its stop, or because there is a short in the switch or the wiring

(2) the alarm system (if you have one) is armed (or has failed and thinks that it is armed) and is not allowing the engine to run

(3) the rev limiter switch has detected an overrev (or, again, has failed) and wants to cut fuel to the engine.

Here are a couple of quick tests:

(1) With the ignition switch off, pull the fuel pump relay. Then turn the switch to ON, and put the relay back in its socket. If you hear it click then one of the above fuel-cut mechanisms is active.

(2) With the ignition switch off, jumper terminals 30 and 87A on the FP relay socket. Then try to start the car. If it starts and runs then you know that the CD box etc are working and that the problem is indeed a fuel-cut situation.

If both of these work as expected then I would start tracking down other options. I am lucky in that my ROW SC has no alarm and no fuel-cut rev limiter so I only have that microswitch to worry about.

The microswitch connector is hard to get to (on the back of the throttle body iIIRC) but if you can reach it and disconnect it, that will take that switch out of the loop. So that is another easy test. Make sure you don't yank on the cold start valve injector instead by accident, they are close to each other back there.

Good luck, let me know if I can offer any other suggestions.

Scott
(glad to use the EE background for something, once in a while)
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Old 09-12-2007, 06:22 AM
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Hi Eric,
Scott has provided a very concise and accurate description of the fuel pump circuit in your '82 SC.
You mentioned no voltage at any of the fuel pump relay socket pins.
The fuel pump fuse F16 gets 12 volts from the ignition switch in the run position. Turn on the key with the fuel pump relay removed. Fuse F16, the fuel pump fuse, should have 12 volts. This 12 volts should also be present at pin 86 and 87A of the fuel pump relay socket from the fuel pump fuse.
The ignition switch also provides 12 volts to pin 87 of the fuel pump relay socket when cranking the engine. Same power as the 12 volts to the starter solenoid (the famous yellow wire).
The ignition switch gets 12 volts directly from the battery via 2 wires about 10 gauge in size. They are actually metric size, close to 10 gauge. Make sure these 2 wires plus a third to the fuses are connected to the battery and are in good shape.
Of course if the 12 volt power is at all of the above, time to chase the fuel pump itself.
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Old 09-12-2007, 08:34 AM
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Scott,
Thanks for all of the information,
It appears that it is the air flow micro switch or the air flow contact at the sensor plate is the culprit, as after I bypassed the FP relay, the car started and ran! Woo Hoo! The only problem that I have found so far is that there is no change of continuity when the air flow sensor plate is raised manually. I did hear a 'click' while putting the FP relay in when the ignition was on so I know that the fuel pump does work.

What is the best way to get to the micro switch so I can rule that out, which would leave me with only a wiring problem (Scary) or a contact issue...

Thanks again for all of your help!
-E
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Old 09-12-2007, 02:53 PM
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The air-plate contact switch is a ground path for the relay and is not "hot". If the air-plate switch fails, it would cause your fuel pump (FP) to run continuously with the ignition on. The only way a failure could occur in the air-plate circuit to disable the FP would be a short in the wiring proximal to the air-plate, not the switch itself. Failure of the rpm limiter switch function could potentially produce the same result. You need to remove the relay and test it directly as this is the most likely failure point.

Here is a link with a post on the wiring and function of the FP and relay:

http://forums.pelicanparts.com/porsche-911-technical-forum/321033-tbitz-efi-msii-post3344108.html#post3344108
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Last edited by sjf911; 09-12-2007 at 03:58 PM..
Old 09-12-2007, 03:51 PM
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KND founding father
 
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OK, so it appears that what you're syaing is that if the relay itself is bad, then lifting the air flow plate would, or could potentially have no effect. I did some rudimentary testing of the relay, and it appears to be good... and I switched the horn relay into the FP slot, and still nothing. The car will run if I bypass the relay by jumping 30 and 87a...What do you suggest I try next?

Thanks again!
-E


And oh yeah, when she fired up it sounded great!
Old 09-12-2007, 04:09 PM
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Eric,
If you think the air flow switch is the culprit, here is a simple test. Similar to your bypass with a jumper wire.
Reach behind the air flow assembly and pull the connector off the air flow switch.
Now start the car as you normally do.
If the car runs with the switch disconnected and does not run with it connected the switch is the problem.
Remember the car runs when the fuel pump relay is NOT energized. You have proven the relay functions by the fact that it clicks on when plugged in and no change when replaced with a known good relay.
Another simple relay and switch test is to have someone feel the fuel pump relay as you lift the air flow plate.
If the relay "clicks off" as you lift the plate and "clicks on" as you lower the plate with the ignition switch in the normal on position the relay and the air flow switch as well as the wires to it are OK.
Do this test quickly, because if the fuel pump in fact comes on, the injectors will be spraying fuel into the cylinders.
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DOUG
'76 911S 2.7, webers, solex cams, JE pistons, '74 exhaust, 23 & 28 torsion bars, 930 calipers & rotors, Hoosiers on 8's & 9's.
'85 911 Carrera, stock, just painted, Orient Red
Old 09-12-2007, 05:18 PM
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I would love to pull the connector, it's just that I can't, for the life of me, find the darn thing. If I could see behind the engine, that would be a different story. Is there an easy way to find it, or am I relegated to feeling for it (my hands aren't the smallest things either)...
Thanks a million,
-E
Old 09-12-2007, 05:23 PM
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CIS Trouble Shooting......

Eric,

If I understood correctly what you had summarized: (1). The car runs when you shunted terminals 30 and 87A. (2). With the ignition switch ON, fuel pump will not run when air plate sensor was lifted. (3). With a good fuel pump relay, car still won't run.

Your problem is terminal 85 (ground). This the line that goes to the air meter switch (sensor plate). Without this ground, the fuel pump relay will be disabled. Send me a PM and I'll show you the connector for sensor plate in the engine harness. Just my two-cents.

Tony
Old 09-12-2007, 05:27 PM
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Remember, terminal 85 can be grounded by either the airplate circuit, factory alarm, or the RPM limiter switch circuit. A short to ground in any of those circuits would disable the FP.
An easy test is to place a continuity tester probe into the terminal 85 socket and ground it to the chassis. If it lights, have someone lift the airplate and see if it breaks. If the light stays on then you definately have a short in one of those circuits. You will then have to try to isolate which.
The other thing to check is if you have a factory or aftermarket alarm system that includes a FP disable. That may complicate isolating the problem.
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Last edited by sjf911; 09-12-2007 at 08:38 PM..
Old 09-12-2007, 08:35 PM
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Eric,

Does your car have a factory alarm system, by any chance? Symptoms sound remarkably like a thread from 5+ years ago, here:

'82 SC Won't run

In that case we eliminated everything else that could have affected the fuel pump relay, and then found a casual reference in the factory service manual to the possibility that the alarm module could disable the car if it failed, and detailed a 'workaround' procedure to get the car running. Not a trivial procedure to say the least, it involves pulling out the fresh air blower to get to the module ... then you can bypass with a 4" - 6" piece of wire and two 0.25" Faston male tabs connecting the two #61 connections together, restoring power to the fuel pump.
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Old 09-12-2007, 10:30 PM
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Good responses.

From the troubleshooting you've done so far it appears that the FP relay is doing its job, but is being activated to cut fuel for some reason, with the likely causes being the air plate switch, the alarm, or the RPM limiter. Or it could be some other wiring fault (harder to find so we will check the other possibilities first).

The alarm and the RPM limiter are both electronic (semiconductor) devices, so they are both very susceptible to damage from seeing reverse polarity power when you had the jumper cables hooked up backwards. Did you have the ignition switch on at the time? If not, I would think the only device that saw the reversed power would have been the alarm (again if you have one). Do you have an alarm?

On the other hand, if you did have the ignition switch on while the cables were connected backwards, the diode in the FP relay would have been conducting all of the available current through the air flow switch contacts in a near-short-circuit. You may have exploded the diode inside the relay (it would still work without it; the black relays are just red relays without diodes) and/or fused the switch contacts closed. Switches usually fail open, but you can burn them closed. (BTDT - my first experience with welding.)

The engine is out of my 78 SC right now, so I will post a picture of the air plate switch connector in a few minutes so you can at least figure out where it is. That won't make the access any easier, but you will know exactly what you can't quite reach.

Another troubleshooting angle would be to look at the connections to terminal 85 of the FP relay socket up at the wiring block to see if there is a junction point there (more than one wire coming in). The Bentley diagram makes it look like the rev limiter wire may join at the relay terminal, but that may not be how the car is built. If you could break the circuit apart you could pinpoint the problem by figuring out which wire is causing the relay to pull.

Scott
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Last edited by scottbooth; 09-13-2007 at 05:52 AM..
Old 09-13-2007, 05:50 AM
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Thanks, I do have a factory alarm system, but it still operates correctly. It seems that this would (may)rule this out.
-E
Old 09-13-2007, 05:59 AM
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Pics of air flow plate switch

Your best bet for access to the air flow plate switch is probably to remove the big rubber CIS boot and try to reach over the body of the venturi.

Excuse the mess (engine, garage, whatever)...here are the pics.

A wide angle view of the engine as seen from the front side of the car. The air flow plate switch is the unplugged blue connector at upper left.




A closer view.




Close up of the connector. Note the wire latch that goes across the top of the connector and is open at the bottom. You need to pry the bottom edges apart and slide the latch up to unplug.




Good luck
Scott
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Old 09-13-2007, 06:30 AM
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Air Meter Switch.....

Eric,

Ground wiring for terminal 85 (fuel pump relay):

This connector is located about 6 inches away from the CDI 6-point connector. The brown wire (on the right) is connected to terminal 85 (fuel pump relay) and goes to the air meter switch (see Scott's picture). The other terminal (air meter switch) goes to alternator at D-. This is where your problem lies (open ground). Improvise a temporary ground connection just to test your fuel pump using this connector. Good luck.

Tony
Old 09-13-2007, 09:43 AM
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Scott,
I was able to reach the connector at the rear of the air flow switch by reaching around from the left (drivers side) and unplugged it. After unplugging it, I experienced the same results, so I reattached it.

Tony,
Maybe I am stupid, but I can't seem to find the connector you were kind enough to post for me. I see a connector with two red and a brown wire going in and one of each coming out on the left side of the engine bay in font of the ignition box, but when disconnected, the engine wouldn't even fire...

Any new ideas?
-E
Old 09-13-2007, 11:08 AM
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I had the same problem with another 911 with factory alarm. I discontected it and the car ran fine. THe alarm still worked normal, but would shut off the fuel pump after the car cranked.
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Old 09-13-2007, 11:36 AM
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Quote:
Originally Posted by Greek55 View Post
I discontected it and the car ran fine. .
Wow...How did you disconnect the alarm? I'll try it as soon as possible!

Thanks,
-E
Old 09-13-2007, 11:40 AM
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Quote:
Originally Posted by Eric M Abramson View Post
Tony,
Maybe I am stupid, but I can't seem to find the connector you were kind enough to post for me. I see a connector with two red and a brown wire going in and one of each coming out on the left side of the engine bay in font of the ignition box, but when disconnected, the engine wouldn't even fire...

Any new ideas?
-E
Eric,

Don't disconnect any wire from that connector. All you need to do is determine where the break in the brown wire is. Not until you establish a good ground with this brown wire, your fuel pump will not run with fuel relay installed unless you hot wired it.

Try to supply ground to terminal 85 and see what happens. Give me a call and I'll walk you through this very basic test. Your current problem is not that complicated. You're just overwhelmed and tired!!!! Relax. We'll fix the problem.

Tony
Old 09-13-2007, 01:04 PM
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